Roughly forty percent of the human genome encodes integral membrane proteins at the cell periphery that provide essential conduits for the communication of extracellular signals to the intracellular milieu. These proteins mediate diverse aspects of cellular homeostasis, serving as central hubs for information transfer within the cell, including nutrient uptake, energy production, hormone signaling, and drug efflux mechanisms that render cancer cells and pathogens resistant to otherwise effective therapeutic treatments of disease. Insights from prior investigations have revealed a pressing need for a deeper understanding of how conformational changes in these essential proteins specify their precise activities and signaling functions. Consequently, there is an urgent demand for new biophysical approaches that enable direct measurements of membrane protein activities from the perspective of motion. The overarching goal of the proposed research is to meet this need through the development of a generalizable approach to directly ascertain the relationship between membrane protein dynamics and activity using single-molecule fluorescence imaging methods. Completion of the proposed Aims will establish a quantitative relationship between transporter dynamics and uptake activity for the first time. It will also provide a critical foundaion for investigating other clinically relevant small-molecule transporters, where a deeper understanding of the relationship between membrane protein dynamics and activity is required to understand how small-molecules modulate their activities. The technological foundations established through this research may also be extended to a broad range of other clinically relevant, integral membrane proteins that do not transport solutes across the membrane. Such proteins include G protein coupled receptors (GPCRs) whose functions hinge upon conformational events that trigger downstream signaling cascades. If successfully enabled, these investigations have the potential to fundamentally change the nature, depth and breadth of investigations that can be achieved within this ubiquitous protein class. Advancements in the technological foundations of single-molecule imaging will ultimately enable investigations of signaling events at the membrane in real time in living cells.